Installation of pipe strings with lengths ranging from 2,000 to 6,000 feet where environmental conditions or other installation constraints impact the structural integrity of the pipe string has long been a problem for pipeline construction engineers. An example of this type of pipeline installation is installation of deep seawater pumping pipelines. Pipelines which are installed to obtain cold biologically inactive seawater from depths of 2,000 feet or below are used in applications such as power generation by ocean thermal energy conversion (OTEC) and mariculture applications requiring cold biologically inactive ocean water which is free of industrial, agricultural, and human pollution and unwanted forms of life.
Deep ocean-water pipelines of this type require a length of approximately 6,000 feet or longer. Such pipes are placed in locations where there is a steep slope of seafloor such as off volcanic islands where the seafloor may reach a depth of 2,000 feet for a 6,000 foot long pipe. Very few such deep ocean-water pipelines have been constructed. One such pipeline was constructed in Cuba during the 1930's by G. Claude for generation of electrical power using thermal difference between surface and deep ocean water based on theories developed by the French physicist D'Arsonval. However, this pipeline was destroyed shortly after installation by a hurricane.
The first such pipeline known to be in continuous service exists at the Natural Energy Laboratory of Hawaii. This prior art installation comprises a 12-inch pipeline moored by anchor bolts to the seafloor in the shallow depths of its length. At greater depths the pipe is held in place by cables connected to battleship anchors at two locations, one of which is near the termination of the pipe. The 12-inch pipeline is constructed of high-density polyethylene (HDPE) and is slightly buoyant even when filled with seawater. The pipeline therefore floats in an inverted catenary position connected between the two battleship anchor and cable arrangements. Installation of this pipe required extensive use of heavy equipment necessary for handling the heavy anchors, and this installation involved operating at sea over a period of several days.
Launching of such pipelines requires assembly and sinking of the pipe. In the prior art, assembly of the pipeline was accomplished using several different techniques. One example technique involved incrementally connecting pipe sections to form subsections of the completed pipe of approximately 2,000 feet in length by installing short pipe sections and floating the increasing length of pipe in the lee of a breakwater for protection from ocean currents and other natural forces such as wave action during assembly. The individual subsections were then towed to sea, aligned and interconnected in the open sea.
Interconnection required the use of large seagoing crane equipment or complex attachment methods to preclude premature flooding and sinking of the pipe. Additionally, the assembly of the subsections and towing of the completed pipe to its final installation location were conducted in the open sea, exposing the pipeline to ocean currents and meteorological conditions with a significant chance of damage or loss.
The cost of installing such pipelines using the prior art techniques is extremely high due to the requirement for tug boats, cranes, and other heavy oceangoing equipment and extensive use of divers for significant periods of time.
A method and apparatus is desired which reduces the cost and risk for installation of such deep water pipes. Avoiding such long-term exposure to ocean forces which may cause structural damage to or loss of a pipe during installation and the elimination of extended time and heavy equipment requirements assists in achieving this goal.